1. Field of the Invention
This invention relates to a novel heat-activatable, pressure-sensitive adhesive tape or sheet (film) which in the normal state has a low stickiness to the touch and thus is poorly bonded to an object, but, upon heating for a short time at a relatively low temperature of at about 50.degree. C. or above, exhibits excellent bond strength since the adhesive reacts on heating and is activated to have a viscosity providing a modulus of elasticity equal to that possessed by known pressure-sensitive adhesives at normal temperature.
2. Description of the Prior Art
A pressure-sensitive adhesive tape comprising a support tape coated with a pressure-sensitive adhesive can be bonded to an object at normal temperature and at a pressure substantially equal to the finger pressure. This type of tape is useful in the protection and ornamentation of objects or in fixing the ends of a wrapping paper. However, if the adhesive layer is continuously subjected to various stresses, flow occurs within the adhesive layer to cause frequent peeling of the adhesive tape from the object. In addition, such flow is more likely to occur under conditions where the tape is exposed to heat and/or moisture. Therefore, the use of a conventional pressure-sensitive adhesive tape has been limited.
The two main types of adhesive tapes that have been developed as an adhesive tape material in which the fluidity of the adhesive after bonding is reduced to a minimum are a thermo-setting adhesive tape material in which a chemical cross-linking reaction is induced, and a heat-activatable adhesive tape material which solidifies after the adhesive is activated by heating. However, to cure the first type of adhesive tape, the adhesive tape must be heated to a temperature higher than 100.degree. C. with a heater that is expensive and the operations required are complicated. In addition, the use of such a tape is limited in that it can be applied to only a heat resistant adherend. The second type of adhesive tape includes a rubber-based heat-activatable adhesive which comprises a mixture of a rubber such as neoprene rubber, nitrile rubber, styrene-butadiene rubber, or natural rubber with a suitable amount of a hydrogenated rosin, a phenolic resin, a polyterpenic resin, etc., and a resin-based heat-activatable adhesive which mainly comprises polyethylene, an ethylene/vinyl acetate copolymer, a polyamide resin, etc. Since the rubber-based adhesive uses a rubber-like elastomer as the main component, the adhesive has low temperature sensitivity and high heat resistance, but when activated upon heating, the adhesive has high melt viscosity and poor "wetting" characteristics and, therefore, it has low bond strength if the surface of the adherend is not smooth. Such defects of the rubber-based adhesive can be overcome with a resin-based adhesive, but the inherent nature of this type of adhesive is that the melt viscosity of the adhesive becomes too low if it is activated by heating. For this reason, the bond strength of this adhesive is markedly reduced upon intermittent or continuous exposure of the bonded adherend to high temperatures (higher than the application temperature).